Optical waveguide modulators used in high-speed optical communications. such as those based on waveguide Mach-Zehnder (MZ) interferometric structures, may require active control of their operating conditions, and in particular of their bias voltage that sets the relative phase of interfering light waves in the modulator in the absence of the modulation signal. The waveguides of the modulator are typically formed in an electro-optic material, for example a suitable semiconductor or LiNbO3, where optical properties of the waveguide may be controlled by applying a voltage. Such a waveguide modulator may be a part of an optical integrated circuit (PIC) implemented in an opto-electronic chip.
Very high speed optical systems may benefit from one of quadrature modulation (QM) formats such as the Quadrature phase shift keying (QPSK) and Quadrature Amplitude Modulation (QAM). These modulation formats may be realized using a quadrature modulator which is typically implemented using nested MZ interferometric structures. For example, a QAM optical signal may be generated by splitting light from a suitable light source between two MZ modulators (MZM) that are synchronously driven by an in-phase (I) modulation signal and a quadrature (Q) modulation signal that carry respective I and Q components of an electrical QAM or QPSK signal, and then combining the resulting I-channel and Q-channel light signals in quadrature, i.e. with a relative optical phase shift ϕIQ equal to 90°, or π/2 radians (rad). For example the two MZMs of a quadrature modulator may each be modulated by a BPSK (binary phase shift keying) signal while being biased at their respective null transmission points for push-pull modulation. When their outputs are added together in quadrature, i.e. with the relative phase shift ϕIQ=π/2, a QPSK signal (Quaternary phase shift keying) results.
At a receiver site, the QM modulated signal may be coherently combined with light from a local oscillator (LO) source, typically using a 90° optical hybrid, which outputs are coupled to one or more differential receivers. The phase of the LO light relative to the received light signal is however typically unknown, and digital signal processing is conventionally used to perform phase recovery and extract the transmitter I-channel and Q-channel signals from the detected electrical signals at the outputs of the differential detectors. This signal processing may be relatively complex and thus typically require relatively power-consuming and expansive digital signal processors.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for demodulating optical quadrature modulated signals.